# How many ion thrusters would be needed to accelerate a 1000 tonne craft at 9.8m/s²?

If we assembled an array of today's best ion thrusters in orbit, (the x3?) how many would be needed to accelerate a 1000 tonne spacecraft at 1g?

I'm thinking Space Tug, for repeated Mars missions. Nuclear powered of course.

• What I can recommend is to first ask how fast any ion thruster could accelerate itself ignoring even a source for electrical power much less a spacecraft. I think it will be surprisingly low acceleration, perhaps even imperceptible at first if you were hanging on wearing a space suit. This is why they are "burned" for months or years to do the same thing that chemical rockets do in seconds or minutes.
– uhoh
Commented Feb 6, 2021 at 8:16
• What about if you used electric motors with fans while you're in atmosphere? Not sure if even that would be realistic but you'd have a better chance of at least getting off the ground Commented Feb 7, 2021 at 21:01
• The same number as the number of people that must push a car, to break the sound barrier with that car. Commented Dec 31, 2021 at 11:17

## You can't do it. It's impossible.

Each thruster provides thrust, but each thruster has mass, as do the power sources needed to power them and the tanks to store their fuel.

No currently existing ion thruster is able to produce anywhere near that much thrust for its mass, and more significantly, even the best power sources (even speculative ones or those at low TRL's) don't provide enough power for the mass to power such an assembly, no matter how large it is. You're looking at milliG's or microG's of acceleration.

More generally, a thruster that was realistically even vaguely practical for accelerating at even fractional G's would almost certainly stretch the definition of "ion thruster" which is originally normally used to refer to gridded ion thrusters which have pretty low limits in thrust to weight ratio.

If you want to accelerate a multi-ton spacecraft at 1 G, then you want chemical rockets, Or if you absolutely insist, possibly an ultra high performance nuclear thermal rocket can be developed (current designs aren't really designed to accelerate at 1 G when mounted on a spacecraft, though they do have greater than one thrust weight ratio).

• My first thought when reading the question: an ion thruster could not lift itself! Excellent explanation!
– Uwe
Commented Feb 6, 2021 at 11:05
• ultra high performance and ultrasafe nuclear thermal rockets please :-)
– uhoh
Commented Feb 6, 2021 at 14:00
• How about the new plasma thruster design described here? The paper says on p. 9-10 that for a simulated plasmoid of radius 10cm the thrust was calculated at about 50 Newtons--no idea what the mass of the apparatus needed to generate a plasmoid like that (or an assembly generating many of them) would be though. Commented Feb 7, 2021 at 6:32
• @cmaster-reinstatemonica - 50 Newtons would accelerate a mass of 5 kg at 10 m/s^2, rather than 5 grams. Still probably too small, but I'm not sure that'd be impossible if each thruster is just a little over 10 cm in cross sectional area--if you imagine an array of 100 such thrusters that would be a little over 1 meter square in area, all powered by the same power source perhaps, then it could be 500 kg or half a metric ton. Commented Feb 7, 2021 at 16:59
• (cont.) Power might be the main problem with keeping the mass that low, the paper says the simulated version of the 10cm plasmoid they looked at would require about 10 MW of power so 100 of them would need 1 GW. But there has been talk about the near-future possibility of compact fusion reactors like the one the Navy filed a patent for, not sure about the ratio of mass to power output they're hoping for. Commented Feb 7, 2021 at 17:03